Car audio question about amps and speaker ohm rating
 

I am looking at an amp that is 75W X 4 @ 4 ohms and 100w X 4 @ 2 ohms. Do all speakers driven by the amp have to be the same phm rating? I was thinking of hooking up a 2 ohm dual voice coil sub and a set of 4 ohm components in the front doors.

Thoughts?

It's fine to have different loads on different (non-stereo) channels. The DVC subs, though, wouldn't you wire them in parallel for a 4 ohm load anyways? Unless the amp is 1 ohm stable....

By connecting one + and one - together on each speaker and connect the remaining 2 terminals to 1 channel do the same for the other sub.. This will give you a single 4 ohm load on each side. Assuming the amp is only 2 ohm stable this is the max you can go. The other 2 channels will go to your component set. Most 4 channel amps have a built in crossover use that.

Jeff

The dual voice coils is so you can send left and right to the same sub right? So each one should be wired seperately.

You could run each coil on a channel but what are you going to do about the component set?

Series wiring of subs increases it's impedance but parallel halves the impedance so 2 2 ohm coils in parallel would be 1 ohm and 2 2 ohm in series would be 4 ohms.

Jeff

Channels 1 and 2 would go to each voice coil on the sub. 3 and 4 would go to the component set.

I suppose I could bridge 1 and 2 and send them to parallel'd voice coils ?

No, If you parallel all VC's you will end up with a .5 ohm load across the briged output, the amp sees half of what it's presented so each side would see a .25 ohm load. You need to series each speaker then run them separately on the rear channels for a 4 ohm load on each side (and the amp will have better control over each speaker. If you run the pair in series again for an 8 ohm load and bridge the amp it will have the same power as single 4 ohm loads on each channel. You can’t parallel the speakers individually because you would have a 1 ohm load on each channel.

Jeff

On another related thought...

What are the amps wired with huge 8, 6, and 4 guage wire? Are those amps really pulling that much current?

Our high current RCs don't use that big of wire?

Use a minimum of 8 gauge. RC cars have very short wire. The longer the distance the larger gauge wire you have to use to handle the power. 8ga is good for about 10 feet and 50 amps continuous. Current draw is basically the RMS output of the amp (all channels) in watts then add about 60% more to that to account for the inefficiency of the amp (unless it’s a class D, then use 40%). So you have a 400 watt amp plus 240W for losses which is 640W of power required. I divide by 12V to end up with 53.3 amps at max power. Amps don’t put out their max power very often and you will probably see about a 15 amp average draw. Also make sure your ground is solid (and same gauge) on clean metal or you will have alternator whine. You didn’t say what the amp was but if it is a quality amp then this is about what you will see. I can tell by the wattage rating that’s not a real high quality amp, and is not 1 ohm stable, but use what you have I guess.

Jeff

They use huge wires to minimize resistance over the ~10' or so from the battey to the amp. Our RC's don't really need wires that large (even though we pull higher currents) because the wires are very short in comparison.

You have to remember a few things about car audio amplifiers:

1: They require high current that has to be delivered over a much longer distance (typically from the front where the battery is, to the trunk where the amp is). Required wire gauge is a function of distance and current. R/C wires are MUCH shorter so can get away with smaller wire.

2: Your typical class A-B amplifier is around 60% efficient. So, if you have a true 500w amp, it is actually pulling 833w from the battery at full-tilt. Assuming a steady 14.4v, that's almost 58A.

3: Related to #1, wire has resistance. The larger the wire, the less the resistance. Resistance creates voltage drop. Let's say a 12GA wire has 0.002 ohms of resistance per foot, and it takes 16 feet to get to the amp. At 100A, that would be a 3.2v drop on just the wire. So, instead of 14.4v getting to the amp, you are getting only 11.2v. Larger gauge wire has less resistance, so there is less voltage drop and the amp runs better.

Got it.

IIRC you are not supposed to hook up a different channel to each voice coil on a DVC sub. The DVC is meant to be used on one single channel and you vary the impedance by wiring in series or parallel like people already mentioned. It's possible that I'm wrong but you might want to check into that.

DVC speakers have been around a long time even before you could bridge car amps. It is perfectly fine to drive a DVC speaker on 2 different channels (provided the impedance is proper). DVC gives you more flexability but it was the only way to drive a single sub back in the day (80's).

Jeff

+1. Also have to make sure to observe polarity or there will be very little output from the sub. :smile:

Generally, the R and L channels carry the same low frequency signal; it's usually only the mids/highs that are seperated for the stereo effect.